Putin isn't worried about NATO, any more than he is concerned about Nazis. That was just the usual Russian smoke-and-mirror act, largely for internal consumption.
Having failed to accomplish the easy win, there's no way to back down and say, "Well, never mind". He still desires the emotional goals of owning that land, and having sold it as an existential risk against NATO, he can continue to get support at home.
Which will last as long as he can continue to staff it with prisoners and disfavored ethnic groups, and various countries continue to fund it by skirting sanctions.
This series was written 8 years ago and predicted another invasion of Ukraine.
The tldr is it was a strategy for Putin to remain in power.
There will be no peace with Putin but a military defeat may be enough to get rid of him. So I don’t really understand at all why Europe and the US are so hesitant when this is a way to obviously damage Putin.